Issue 7, 2018

A comparative synthesis of ZSM-5 with ethanol or TPABr template: distinction of Brønsted/Lewis acidity ratio and its impact on n-hexane cracking

Abstract

Two kinds of ZSM-5 zeolites with various SiO2/Al2O3 ratios were synthesized using ethanol (EtOH) or tetrapropylammonium bromide (TPABr) as the template respectively and their physicochemical properties were investigated extensively. Different ZSM-5 morphologies were observed for each kind of sample, the former with coffin-shaped single crystals and the latter with aggregates assembled from nanocrystals. The desilication process occurred and intensified at a later period of crystallization for ZSM-5 by EtOH with increasing SiO2/Al2O3 ratio and/or increasing alkalinity, due to its weaker pore-filling effect. Specifically, ZSM-5 zeolites by EtOH had a large amount of Brønsted acid sites (BAS) and a much less amount of Lewis acid sites (LAS), leading to an ultra-high B/L ratio (45–66); while those by TPA+ with comparable acidity possessed a usual B/L ratio (6–8). Owing to no octahedrally coordinated Al in both kinds of ZSM-5, the amount of LAS should be attributed to the tri-coordinated Al “defect” sites, in accordance with the extra silanols determined by OH-IR. Further, ZSM-5 by EtOH exhibited fewer internal defective Al sites, which was responsible for its small number of LAS. Applied to n-hexane cracking, the ZSM-5 zeolite by EtOH had high activity with twice the lifetime (60 vs. 35 h) compared to that by TPA+ with a similar SiO2/Al2O3 ratio. The ratio of accumulated amount of coke to converted n-hexane was obtained as a measure of average coke selectivity (Sc). The ZSM-5 zeolites by EtOH displayed less than half the Sc value (0.45) in comparison with those by TPA+ (1.08), due to the big difference in LAS, regardless of their SiO2/Al2O3 ratio, morphological structure and reaction conditions. Thus, for ZSM-5 possessing a high B/L ratio improves the catalytic activity and decreases coke deposit through suppressing the dehydrogenation process of the reaction intermediate to form coke precursor.

Graphical abstract: A comparative synthesis of ZSM-5 with ethanol or TPABr template: distinction of Brønsted/Lewis acidity ratio and its impact on n-hexane cracking

Supplementary files

Article information

Article type
Paper
Submitted
27 Nov 2017
Accepted
25 Feb 2018
First published
27 Feb 2018

Catal. Sci. Technol., 2018,8, 1923-1935

A comparative synthesis of ZSM-5 with ethanol or TPABr template: distinction of Brønsted/Lewis acidity ratio and its impact on n-hexane cracking

T. Ma, L. Zhang, Y. Song, Y. Shang, Y. Zhai and Y. Gong, Catal. Sci. Technol., 2018, 8, 1923 DOI: 10.1039/C7CY02418E

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements